OSLO (Reuters) - Scientists based in the United States won the lion's share of this year's Nobel prizes in science and economics but all of them were immigrants, making literature laureate Bob Dylan the only U.S.-born winner of 2016.

Of the nine winners in medicine, physics, chemistry and economics, six work at U.S. universities. Five of those were born in Britain and one in Finland, making this the first year since 1999 that no American-born scientist has claimed a Nobel.

The roll of honor highlights the pre-eminence of the United States in attracting the world's most brilliant scientists.

According to one of the British winners, it also reflects the political climate that drove many scientists to leave his home country a generation ago.

"This seems a historical phenomenon. At least in the late 1970s and the 1980s the UK seems to have had a 'brain drain',” Duncan Haldane, who shared the Physics Prize for work at Princeton, told Reuters.

He said some scientists had been deterred by a stress in Britain at the time on "useful research" with economic spinoffs.

By contrast, U.S. promotion of "curiosity-driven" research with less clear goals "attracted scientists from around the world" and led to many unexpected breakthroughs, he said.

IMMIGRATION DEBATE

Atif Mian, a professor of economics at Princeton who was born in Pakistan, said the science prizes were a reminder of the value of immigration when many voters, from the United States to Britain, are demanding tighter controls.

"Science has really benefited the United States," he said. "Universities like Princeton have become much more competitive. The net is cast much wider, not just internationally but within the United States."

Wolfgang Ketterle, a German-born Nobel Physics laureate from 2001 who works at the Massachusetts Institute of Technology (MIT), told Reuters: "U.S. universities continue to attract talent from all countries. But there is more competition now with universities in Europe and China."

Reflecting that, an annual league table of the world's top universities last month showed Oxford displacing the California Institute of Technology, leader for the past five years, as number one, while Chinese universities climbed. But Oxford's vice-chancellor said rivals were looking to poach some of its top academics amid the uncertainty surrounding Britain's exit from the European Union.

CHANGING PATTERNS

The United States has won more Nobels than any other country since the awards were first made in 1901, but it did not dominate from the start.

"Before the Second World War, Germany was the country that got most Nobel Prizes," said Gustav Kallstrand, the curator of the Nobel Museum in Stockholm. But the rise of the Nazis drove scientists including Albert Einstein to the United States.

Among recent hints of change, Tu Youyou, who shared the Medicine Prize last year, was the first Chinese science laureate for work done in China, he said. She won for her research on malaria.

In 2016, the Physics Prize went to a British-born trio - Haldane, David Thouless of the University of Washington and Michael Kosterlitz of Brown University - for work revealing unusual states of matter that have led to advances in electronics.

J. Fraser Stoddart, born in Edinburgh and based at Northwestern University, was one of three Chemistry laureates for developing tiny molecular machines. Britain's Oliver Hart of Harvard and Finland's Bengt Holmstrom of MIT shared the economics prize.